Messages in this thread Patch in this message | | | Date | Thu, 9 Jun 2022 23:23:07 +0800 | From | Tao Zhou <> | Subject | Re: [PATCH v10 1/7] sched/fair: Provide u64 read for 32-bits arch helper |
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On Tue, Jun 07, 2022 at 01:32:48PM +0100, Vincent Donnefort wrote:
> From: Vincent Donnefort <vincent.donnefort@arm.com> > > Introducing macro helpers u64_u32_{store,load}() to factorize lockless > accesses to u64 variables for 32-bits architectures. > > Users are for now cfs_rq.min_vruntime and sched_avg.last_update_time. To > accommodate the later where the copy lies outside of the structure > (cfs_rq.last_udpate_time_copy instead of sched_avg.last_update_time_copy), > use the _copy() version of those helpers. > > Those new helpers encapsulate smp_rmb() and smp_wmb() synchronization and > therefore, have a small penalty for 32-bits machines in set_task_rq_fair() > and init_cfs_rq(). > > Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com> > Signed-off-by: Vincent Donnefort <vdonnefort@google.com> > Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 77b2048a9326..05614d9b919c 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -612,11 +612,8 @@ static void update_min_vruntime(struct cfs_rq *cfs_rq) > } > > /* ensure we never gain time by being placed backwards. */ > - cfs_rq->min_vruntime = max_vruntime(cfs_rq->min_vruntime, vruntime); > -#ifndef CONFIG_64BIT > - smp_wmb(); > - cfs_rq->min_vruntime_copy = cfs_rq->min_vruntime; > -#endif > + u64_u32_store(cfs_rq->min_vruntime, > + max_vruntime(cfs_rq->min_vruntime, vruntime)); > } > > static inline bool __entity_less(struct rb_node *a, const struct rb_node *b) > @@ -3313,6 +3310,11 @@ static inline void cfs_rq_util_change(struct cfs_rq *cfs_rq, int flags) > } > > #ifdef CONFIG_SMP > +static inline u64 cfs_rq_last_update_time(struct cfs_rq *cfs_rq) > +{ > + return u64_u32_load_copy(cfs_rq->avg.last_update_time, > + cfs_rq->last_update_time_copy); > +} > #ifdef CONFIG_FAIR_GROUP_SCHED > /* > * Because list_add_leaf_cfs_rq always places a child cfs_rq on the list > @@ -3423,27 +3425,9 @@ void set_task_rq_fair(struct sched_entity *se, > if (!(se->avg.last_update_time && prev)) > return; > > -#ifndef CONFIG_64BIT > - { > - u64 p_last_update_time_copy; > - u64 n_last_update_time_copy; > - > - do { > - p_last_update_time_copy = prev->load_last_update_time_copy; > - n_last_update_time_copy = next->load_last_update_time_copy; > - > - smp_rmb(); > - > - p_last_update_time = prev->avg.last_update_time; > - n_last_update_time = next->avg.last_update_time; > + p_last_update_time = cfs_rq_last_update_time(prev); > + n_last_update_time = cfs_rq_last_update_time(next); > > - } while (p_last_update_time != p_last_update_time_copy || > - n_last_update_time != n_last_update_time_copy); > - } > -#else > - p_last_update_time = prev->avg.last_update_time; > - n_last_update_time = next->avg.last_update_time; > -#endif > __update_load_avg_blocked_se(p_last_update_time, se); > se->avg.last_update_time = n_last_update_time; > } > @@ -3796,12 +3780,9 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq) > } > > decayed |= __update_load_avg_cfs_rq(now, cfs_rq); > - > -#ifndef CONFIG_64BIT > - smp_wmb(); > - cfs_rq->load_last_update_time_copy = sa->last_update_time; > -#endif > - > + u64_u32_store_copy(sa->last_update_time, > + cfs_rq->last_update_time_copy, > + sa->last_update_time); > return decayed; > } > > @@ -3933,27 +3914,6 @@ static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s > } > } > > -#ifndef CONFIG_64BIT > -static inline u64 cfs_rq_last_update_time(struct cfs_rq *cfs_rq) > -{ > - u64 last_update_time_copy; > - u64 last_update_time; > - > - do { > - last_update_time_copy = cfs_rq->load_last_update_time_copy; > - smp_rmb(); > - last_update_time = cfs_rq->avg.last_update_time; > - } while (last_update_time != last_update_time_copy); > - > - return last_update_time; > -} > -#else > -static inline u64 cfs_rq_last_update_time(struct cfs_rq *cfs_rq) > -{ > - return cfs_rq->avg.last_update_time; > -} > -#endif > - > /* > * Synchronize entity load avg of dequeued entity without locking > * the previous rq. > @@ -6960,21 +6920,8 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu) > if (READ_ONCE(p->__state) == TASK_WAKING) { > struct sched_entity *se = &p->se; > struct cfs_rq *cfs_rq = cfs_rq_of(se); > - u64 min_vruntime; > - > -#ifndef CONFIG_64BIT > - u64 min_vruntime_copy; > - > - do { > - min_vruntime_copy = cfs_rq->min_vruntime_copy; > - smp_rmb(); > - min_vruntime = cfs_rq->min_vruntime; > - } while (min_vruntime != min_vruntime_copy); > -#else > - min_vruntime = cfs_rq->min_vruntime; > -#endif > > - se->vruntime -= min_vruntime; > + se->vruntime -= u64_u32_load(cfs_rq->min_vruntime); > } > > if (p->on_rq == TASK_ON_RQ_MIGRATING) { > @@ -11422,10 +11369,7 @@ static void set_next_task_fair(struct rq *rq, struct task_struct *p, bool first) > void init_cfs_rq(struct cfs_rq *cfs_rq) > { > cfs_rq->tasks_timeline = RB_ROOT_CACHED; > - cfs_rq->min_vruntime = (u64)(-(1LL << 20)); > -#ifndef CONFIG_64BIT > - cfs_rq->min_vruntime_copy = cfs_rq->min_vruntime; > -#endif > + u64_u32_store(cfs_rq->min_vruntime, (u64)(-(1LL << 20))); > #ifdef CONFIG_SMP > raw_spin_lock_init(&cfs_rq->removed.lock); > #endif > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > index 1f97f357aacd..bf4a0ec98678 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -520,6 +520,45 @@ struct cfs_bandwidth { }; > > #endif /* CONFIG_CGROUP_SCHED */ > > +/* > + * u64_u32_load/u64_u32_store > + * > + * Use a copy of a u64 value to protect against data race. This is only > + * applicable for 32-bits architectures. > + */ > +#ifdef CONFIG_64BIT > +# define u64_u32_load_copy(var, copy) var > +# define u64_u32_store_copy(var, copy, val) (var = val) > +#else > +# define u64_u32_load_copy(var, copy) \ > +({ \ > + u64 __val, __val_copy; \ > + do { \ > + __val_copy = copy; \ > + /* \ > + * paired with u64_u32_store, ordering access \
s/u64_u32_store/u64_u32_store_copy()/
> + * to var and copy. \ > + */ \ > + smp_rmb(); \ > + __val = var; \ > + } while (__val != __val_copy); \ > + __val; \ > +}) > +# define u64_u32_store_copy(var, copy, val) \ > +do { \ > + typeof(val) __val = (val); \ > + var = __val; \ > + /* \ > + * paired with u64_u32_load, ordering access to var and \
s/u64_u32_load/u64_u32_load_copy()/
> + * copy. \ > + */ \ > + smp_wmb(); \ > + copy = __val; \ > +} while (0)
Try again here from me. The semantics of this macro is different from the original code. To be consistent with the original code, the 'copy = __val;' should be changed to 'copy = var'. Why not original code here.
They are different in that the var can be changed currently on another CPU and copy can be different from __val.
Documentation/memory-barriers.txt(line 1351, 5.19.0-rc1-rt1): In section: (*) What are memory barriers? - Multicopy atomicity. The example there can be referenced here I think.
And I feel that here the smp_wmb() should be changed to smp_mb(). One user of this macro have the load/write write. Another user have write write if I am not wrong.
Also,
'Memory Barriers: a Hardware View for Software Hackers' (July 23, 2010) Section: 6.3 Example 2, I just scrolled to the code in table 3 and check that the semantics there is almost the same with the semantics here.
I do not have confidence to convince anything just after some days when I read it.
According to those two examples, if the small penalty is small, this u64_u32_store_copy() macro should be changed like this:
# define u64_u32_store_copy(var, copy, val) \ do { \ typeof(val) __val = (val); \ var = __val; \ /* \ * paired with u64_u32_load_copy(), ordering access to var and \ * copy. \ */ \ smp_mb(); \ copy = var; \ } while (0)
Not sure and this is the continuation to my reply to this patch in v4.
Thanks, Tao > +#endif > +# define u64_u32_load(var) u64_u32_load_copy(var, var##_copy) > +# define u64_u32_store(var, val) u64_u32_store_copy(var, var##_copy, val) > + > /* CFS-related fields in a runqueue */ > struct cfs_rq { > struct load_weight load; > @@ -560,7 +599,7 @@ struct cfs_rq { > */ > struct sched_avg avg; > #ifndef CONFIG_64BIT > - u64 load_last_update_time_copy; > + u64 last_update_time_copy; > #endif > struct { > raw_spinlock_t lock ____cacheline_aligned; > -- > 2.36.1.255.ge46751e96f-goog
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